Metal gate electrodes are currently being investigated to replace polysilicon gate electrodes in today's ever shrinking and changing transistor devices. One of the principal reasons the industry is investigating replacing polysilicon gate electrodes with metal gate electrodes is to solve polysilicon depletion effects for future CMOS devices. Traditionally, polysilicon gate electrodes with overlying silicides have been used for the gate electrodes in CMOS devices. However, as device feature sizes continue to shrink, polysilicon depletion is becoming a serious issue.
Accordingly, metal gates have been proposed. However, in order to optimize the threshold voltage (Vt) in CMOS devices, metals with the appropriate work functions (e.g., from the silicon valence band to that of the conduction band) are required. For instance, the metal gates need tunable work functions for NMOS and PMOS devices similar to present polysilicon gate technology, requiring the work functions of metal gates to range from 4.1˜4.4 eV for NMOS and 4.8˜5.2 eV for PMOS (see, B. Cheng, B. Maiti, S. Samayedam, J. Grant, B. Taylor, P. Tobin, J. Mogab, IEEE Intl. SOI Conf. Proc., pp. 91–92, 2001).
Unfortunately, both material and processing issues arise while attempting to manufacture metal gates having different work functions. In one example, a different metal gate material is used for the NMOS devices and the PMOS devices. Unfortunately, the integration of two different metals in a CMOS device flow is difficult, for example requiring significant changes to the CMOS process flow. Ideally, a single metal layer would be preferred, with a portion of the single metal layer being exposed to a specific process to adjust the work function of the exposed portion. This, however, is a difficult technique to perfect.
Accordingly, what is needed is a method for manufacturing metal gate structures for CMOS devices that begins with a single metal layer and adjusts a portion of the single metal layer to have a different work function than the remaining portion, without experiencing the drawbacks of the prior art methods.